The present invention relates to an optical communication system provided with an optical fiber amplifier and, more particularly, to an optical communication system provided with an optical fiber amplifier constituted of a rare-earth-doped fiber which is with a rare earth element.
Optical amplifiers which are capable of amplifying an optical signal directly without using an electric circuit, are being intensively studied in many research institutions as a key device in the optical communication system in the future because of their characteristic, readiness to provide large capacity, and capability of amplifying multiple channels en bloc. Optical communication systems including such optical amplifiers have been proposed such that employ the optical amplifier as an optical power booster for compensating for branching or insertion loss and increasing the transmission power, that use the optical amplifier as an optical preamplifier for improvement of reception sensitivity, that use the optical amplifier as an optical repeater whereby miniaturization of the repeater is achieved and reliability on the repeater is enhanced, and so on. Research is being conducted for optimized arrangement of such systems.
Optical amplifiers that are the objects of the research conducted so far are broadly classified into: (a) optical amplifiers using an optical fiber doped with a earth element such as Er, Nd, and Yb (hereinafter referred to as "rare-earth-doped fiber", this term covering a wide range of waveguide structures including a waveguide doped with a rare earth element); (b) optical amplifiers of a semiconductor laser type; and (c) optical amplifiers making use of the non-linearity within an Of these, the optical amplifier (a) above, i.e., that uses a rare-earth-doped fiber, has advantageous characteristics such that it has no dependency on polarization, produces low noise, and incurs a small loss at its coupling with the transmission line.
When an optical amplifier is used as an optical repeater, a supervisory control function for the optical amplifier is indispensable. As a supervisory the optical amplifier (b) above, i.e., the one being of a semiconductor laser type, there is known a system disclosed, for example, in Ellis, A. D. et al.: Supervisory system for cascaded semiconductor laser amplifier repeaters, Electron. Lett., Vol. 25, No. PP. 309-311 (Mar. 2, 1989). However, since the for the semiconductor laser-type optical amplifier is such that the injection current to the semiconductor laser type optical amplifier is detected therein, the same system as above cannot be applied to the optical communication system provided with an optical fiber amplifier. In other words, there is found no prior art supervisory control system suitable for optical fiber amplifiers.